On-line math calculation
I know several nice Math software packages, mostly for Windows (Derive, Mathematica, MathCAD, Matlab ...). My university has licenses for some of them but I am not always on campus, using Windows, when I need some help. There are some free math programs too (Scilab, Octave, Maxima, Axiom ...).
What it is not always known is that there is an interesting set of on-line calculators and solvers that sometimes can help you get the job done without installing anything. Quite interesting when you are not even using your own computer.
For easy calculations and units conversion Google calculator is a neat tool. How do you use it? Just type your calculation on Google search box (i.e. "5/sin(32)" or "25 celsius in farenheit" ). Really worthwhile to convert between American and European fuel economy units!
But maybe you already have a calculator so this Google feature while nice does more or less the same. You may need to do something more advanced like online matrix calculation. I have found the linked page very useful for my son to check his homework.
Today I needed to solve some transcendental equations and I have found the online version of Yacas quite useful as one of the samples is a Newton iterative solver. You need Java plug-in though.
There are plenty of other sites I've used in the past but today these are the ones I've used recently.
Oops, I've almost forgotten the cool integrator by Wolfram Research. And also an on-line derivative calculator.